range_enc_arfcns: avoid runtime error on zero size

If size <= 1, avoid allocating arfcns_left[size / 2], which results in a zero
size and causes, with gcc 7.3.0 sanitizer, runtime errors:

../../../../src/osmo-bsc/src/libbsc/arfcn_range_encode.c:95:6: runtime error: variable length array bound evaluates to non-positive value 0
../../../../src/osmo-bsc/src/libbsc/arfcn_range_encode.c:96:6: runtime error: variable length array bound evaluates to non-positive value 0

This fixes some of the errors of gsm0408_test, as revealed by a sanitizer build
using gcc (Debian 7.3.0-12) 7.3.0.

Change-Id: Idab2a194fb9d7c41ed3367f935080eaae4ce367f
1 file changed